OFDM channel estimation in the presence of interference

ABSTRACT

Systems and methods for estimating channel response in the presence of interference. Interference and/or noise present on received training symbols is estimated. Based on the measured noise and/or interference, a weighting among training symbols is developed. Channel response is then estimated based on a weighted least squares procedure.

RELATED PATENT APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 10/263,454 filed on Oct. 2, 2002 now U.S. Pat. No. 7,010,049titled OFDM CHANNEL ESTIMATION IN THE PRESENCE OF INTERFERENCE. Thecontents of U.S. patent application Ser. No. 10/263,454 are incorporatedherein by reference.

U.S. patent application Ser. No. 10/263,454 is in turn a continuation ofU.S. patent application Ser. No. 09/410,945 filed on Oct. 4, 1999 titledOFDM CHANNEL ESTIMATION IN THE PRESENCE OF INTERFERENCE (now U.S. Pat.No. 6,487,253). The contents of U.S. patent application Ser. No.09/410,945 are incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present application relates to digital communications and moreparticularly to systems and methods for estimating the response of achannel between two nodes of a communication network.

Orthogonal frequency division multiplexing (OFDM) systems offersignificant advantages in many real world communication systems,particularly in environments where multipath effects impair performance.OFDM divides the available spectrum within a channel into narrowsubchannels. In a given so-called “burst,” each subchannel transmits onedata symbol. Each subchannel therefore operates at a very low data ratecompared to the channel as a whole. To achieve transmission inorthogonal subchannels, a burst of frequency domain symbols areconverted to the time domain by an IFFT procedure. To assure thatorthogonality is maintained in dispersive channels, a cyclic prefix isadded to the resulting time domain sequence. The cyclic prefix is aduplicate of the last portion of the time domain sequence that isappended to the beginning. To assure orthogonality, the cyclic prefixshould be at least as long as the duration of the impulse response ofthe channel.

To maximize the performance of an OFDM system, it is desirable that theresponse of the channel be known at the receiver end of the link. Toprovide the receiver with knowledge of the channel response, thetransmitter typically includes training symbols as part of the frequencydomain burst. The training symbols have known values when transmittedand their values as received may be used in determining the channelresponse.

One technique for estimating channel response based on received trainingsymbol values is disclosed in WO 98/09385, the contents of which areherein incorporated by reference. A modification of this channelestimation technique that takes into account channel components havingknown response is disclosed in U.S. application Ser. No. 09/234,929, thecontents of which are herein incorporated by reference.

Typically, the training symbols are interspersed among the data symbolsin the frequency domain burst. A limited number of such training symbolsare sufficient to characterize the overall channel response. A problemarises, however, if a narrow band interferer signal corrupts receptionof a particular training symbol. The value of that training symbol asreceived will then reflect not only the channel response but also theinterference. This will cause the channel estimation procedure tomisestimate the channel response at the training symbol position and atsurrounding data symbol positions within the frequency domain.

What is needed is a technique that will provide improved estimation ofchannel response in an OFDM system in the presence of interference thatcorrupts transmission of training information.

SUMMARY OF THE INVENTION

Systems and methods for estimating channel response in the presence ofinterference are provided by virtue of the present invention.Interference and/or noise present on received training symbols isestimated. Based on the measured noise and/or interference, a weightingamong training symbols is developed. Channel response is then estimatedbased on a weighted least squares procedure.

According to one aspect of the present invention, a method forestimating a channel response in a digital communication systemincludes: receiving a time domain OFDM burst, converting the time domainOFDM burst to a frequency domain OFDM burst, extracting a vector oftraining symbols having known transmitted values from the frequencydomain OFDM burst, determining weights for the training symbols based onmeasured noise and/or interference, and using the weights and thetraining symbols to estimate the channel response.

A further understanding of the nature and advantages of the inventionsherein may be realized by reference to the remaining portions of thespecification and the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a system for receiving OFDM signals according to oneembodiment of the present invention.

FIG. 2 is a graph of channel response magnitude over frequency depictingthe effects of interference on a channel response estimate.

FIG. 3 is a top level diagram of channel estimation processing accordingto one embodiment of the present invention.

FIG. 4 is a top level flowchart describing steps of estimating a channelresponse in the presence of interference according to one embodiment ofthe present invention.

FIG. 5 is a flowchart describing detailed steps of estimating channelresponse in the presence of interference according to one embodiment ofthe present invention.

DESCRIPTION OF SPECIFIC EMBODIMENTS

The present invention will be described in reference to the use of OFDM(Orthogonal Frequency Division Multiplexing) for communication of data.In OFDM, the available bandwidth is effectively divided into a pluralityof subchannels that are orthogonal in the frequency domain. During agiven symbol period, the transmitter transmits a symbol in eachsubchannel. To create the transmitted time domain signal correspondingto all of the subchannels, an IFFT is applied to a series of frequencydomain symbols to be simultaneously transmitted, a “burst.” Theresulting series of time domain symbols is augmented with a cyclicprefix prior to transmission. The cyclic prefix addition process can becharacterized by the expression:[z(1) . . . z(N)]^(T)

[z(N−μ+1) . . . z(N)z(1) . . . z(N)]^(T)

On the receive end, the cyclic prefix is removed from the received timedomain symbols. An FFT is then applied to recover the simultaneouslytransmitted frequency domain symbols. The cyclic prefix has length μwhere μ is greater than or equal to a duration of the impulse responseof the overall channel and assures orthogonality of the frequency domainsubchannels.

There are other ways of simultaneously transmitting a burst of symbolsin orthogonal channels or substantially orthogonal channels including,e.g., use of the Hilbert transform, use of the wavelet transform, usinga batch of frequency upconverters in combination with a filter bank,etc. Wherever the term OFDM is used, it will be understood that thisterm includes all alternative methods of simultaneously communicating aburst of symbols in orthogonal or substantially orthogonal subchannels.The term frequency domain should be understood to refer to any domainthat is divided into such orthogonal or substantially orthogonalsubchannels.

FIG. 1 depicts a system 100 for receiving OFDM signals carrying embeddedtraining information according to one embodiment of the presentinvention. RF carrier signals modulated with successive time domain OFDMbursts are received via an antenna 102. A receiver system 104 filtersand amplifies the received RF signals, converts the RF signals to anintermediate frequency (IF), filters and amplifies the IF signals,downconverts the IF signal to baseband, and performs further basebandprocessing including converting the analog baseband signal to a digitalsignal representing a series of time domain OFDM bursts.

The time domain OFDM bursts are converted to the frequency domain by aFFT stage 106. The training symbols are extracted from each burst by atraining symbol extraction block 108. Each frequency domain OFDM burstincludes both training symbols and data symbols. The transmitted valuesof the training symbols are known by a channel estimation block 110.Based on the values of the extracted training symbols, channelestimation block 110 estimates the channel response experienced by eachburst. Each channel estimate response consists of a complex channelresponse value for each frequency domain symbol position. Anequalization block 112 corrects the received value for each frequencydomain data symbol by dividing the received value by the channelresponse value at its frequency domain position. A decoding stage 114decodes any channel codes applied by the transmitter including but notlimited to, e.g., convolutional coding, trellis coding, Reed-Solomoncoding, etc.

The values of the training symbols as received will be affected not onlyby the channel response but also by noise and/or interference. FIG. 2depicts channel response magnitude over frequency within a burst.Training symbols 202 are spaced periodically throughout the burst. Ascan be seen, the received values of training symbols 202 will beaffected by the channel response. The second training symbol from theleft is also, however, affected by the presence of a narrow bandinterferer coincident with that training symbol's frequency position.Its value as received is marked by a designator 204. The received valueincludes the effects of the narrow band interference. Employing theprior art channel estimation techniques, the corrupted received trainingsymbol value 204 will affect the channel response estimate over a rangebetween delimiters 206. The present invention provides systems andmethods for reducing the effects of narrow band interferers superimposedon training symbols.

FIG. 3 is a top level view of channel estimation processor 110 accordingto one embodiment of the present invention. For each received OFDMfrequency domain burst, received training symbols T_(l) through T_(v)are input to an IFFT/interference processing block 302. Block 302estimates a channel impulse response based on a measurement of the noiseand interference present on the training symbols and a weighted leastmean square procedure. There are v training symbols input to block 302for each burst and block 302 generates v symbols as part of an impulseresponse estimate or possibly fewer than v symbols if certain ones ofthe training symbols are nulled due to their corruption by interferenceand/or noise. IFFT/interference processing block 302 preferably smoothesthe impulse response estimate.

An FFT block 304 pads the end of the smoothed impulse response output byblock 302 with 0's to extend the response to length N where N is thesystem burst length. FFT block 304 then applies the FFT to the zeropadded impulse response estimate to obtain the channel responseestimate.

FIG. 4 is a flowchart describing steps of the operation of channelestimation processor 110 as depicted in FIG. 3. The estimation of noiseand interference assumes that channel response will change slowlyrelative to OFDM burst rate and that therefore successive channelresponse values for each frequency domain symbol position will be highlycorrelated. For each received frequency domain symbol including eachtraining symbol, the following expression will hold true:X(n,k)=H(n,k)

(n,k)+W(n,k)

where n denotes a frequency domain symbol position, k identifies eachburst in chronological order, X denotes a received frequency domainsymbol value, H refers to a channel response value, Z refers to a valueas transmitted, and W refers to combined noise and/or interferencesuperimposed on a particular frequency domain symbol. For, n∈J where Jis the set of frequency domain symbol positions allocated for use bytraining symbols, one can define a quantity:

${{V( {n,k} )}\;\underset{\underset{\_}{\_}}{\Delta}\;\frac{X( {n,k} )}{( {n,k} )}} - \frac{X( {n,{k + 1}} )}{( {n,{k + 1}} )}$

Because the channel response values are assumed to change slowly, onecan then infer that

${{V( {n,k} )}\;\underset{\underset{\_}{\_}}{\Delta}\;\frac{W( {n,k} )}{( {n,k} )}} - \frac{W( {n,{k + 1}} )}{( {n,{k + 1}} )}$

The quantity V(n,k) may be statistically characterized over time foreach frequency domain position n∈J to find.

${\sigma_{v}^{2}(n)} = {\underset{k}{E}\;{{{V\mspace{11mu}( {n,k} )}}^{2}.}}$For example, one could average V(n,k) over successive bursts by:σ_(v) ²(n,k+1)=β|v(n,k)|²+(1−β)σ_(v) ²(n,k)∀n

The combined noise and/or interference energy,

${{\sigma_{W}^{2}(n)} = {\underset{k}{E}\;{{v\mspace{11mu}( {n,k} )}}^{2}}},$will then be found by:

${\sigma_{W}^{2}(n)} = {\frac{{\sigma_{v}^{2}(n)}\;{{(n)}}^{2}}{2}.}$

Once the noise and/or interference for each training symbol has beenestimated by time averaging, a weighting among the training symbols isdetermined based on the measured noise and/or measured interference atstep 404. The weighting is such that training symbols experiencinggreater corruption by interference and/or noise will have less influencein determining the channel response estimate. The weighting may becharacterized by a matrix R having dimensions v by v.

In one embodiment, the weighting is implemented by an effective nullingof one or more training symbols whose received values have beencorrupted by interference. For example, one may always null the singletraining symbol that is most corrupted by interference. Alternatively,one may null the most corrupted training symbol only if the noise and/orinterference energy on that symbol exceeds the threshold. Weightingmatrix R then has the value 1 at each position along its diagonalcorresponding to a training symbol that has not been nulled, the value 0at each position along its diagonal corresponding to a nulled trainingsymbol position, and 0 at all matrix positions off the diagonal. Ifgreater complexity can be tolerated, a more exact channel response maybe estimated by setting the values of weighting matrix R to be the σ_(W)²(n) values for each training symbol position along the diagonal andzero elsewhere.

At step 406, a weighted least squares procedure is used to estimate thechannel impulse response. This procedure takes into account theweighting matrix R determined at step 404 to arrive at an estimate thatconsiders interference. The expressionh _(wls)=(Y _(v) ^(*) RY _(v))⁻¹ Y _(v) ^(*) RĤmay be used to determine the weighted least squares impulse responsewhere Ĥ is a vector consisting of each received training symbol dividedby the known transmitted value, R is the weighting matrix describedabove, and Y_(v) represents the v-point FFT matrix with elements

${Y_{v}( {n,m} )} = {\frac{1}{\sqrt{v}}\;{\mathbb{e}}^{j\; 2\;\pi\; n\;{m/v}}}$where n and m vary between 0 and v−1.

In the embodiment that nulls certain training symbols to implementweighting, the weighted least square estimation procedure may besimplified. The simplified estimation procedure is described withreference to FIG. 5.

The impulse response is preferably smoothed by using an expression suchas:h(k+1)=γh _(wls)+(1−γ)h(k)

At step 408, FFT block 304 zero pads the smoothed impulse response andtakes the FFT of the result to determine the channel response estimatethat can then be used for equalization.

FIG. 5 is a flowchart describing a simplified procedure for applying aweighted least means square procedure to identify the impulse response.The procedure of FIG. 5 assumes that p training symbols are to be nulledby application of the weighting matrix R where p is greater than orequal to 1.

At step 502, the weighting matrix R is applied to the vector Ĥ by use ofthe following expression:X=RĤwhere X is a vector having v complex components.

At step 504, the IFFT of the result of step 502 is obtained but the lastp terms of this IFFT are discarded. This process may be characterized bythe following expressions:

$\begin{matrix}{\overset{\_}{h} = {\lbrack I_{v - p} \rbrack\;{Y_{v}^{*}( \overset{\_}{X} )}}} \\{= {Y_{v}^{*}\overset{\_}{X}}}\end{matrix}$where h is a vector having v−p complex components and Y_(v) ^(*)represents the v-point IFFT matrix with elements

${Y_{v}( {n,m} )} = {\frac{1}{\sqrt{v}}\;{\mathbb{e}}^{j\; 2\;\pi\; n\;{m/v}}}$where n and m vary between 0 and v−1.

At step 506, an expression β is found by zero padding the result of step504 to include v elements and then taking the FFT but obtaining onlythose entries corresponding to training symbols that have been nulled.This process may be characterized by the expressionβ=Y_(t) ^(*)[₀ ^(h) ]

where β is a vector having p components, and

where Y_(t) ^(*) represents the rows of the v by v IFFT matrixcorresponding to the positions of the training tones that have beennulled.

At step 508, an expression α is derived as follows:α=Q⁻¹βQ=vI _(p) −Y _(t) ^(*) Y _(t)

where α is a vector having p complex elements, and where

where I_(p) is the identity matrix having p rows and p columns.

Then at step 510, the impulse response is found by applying theexpression:h _(wls) = h+Y _(t)α

It will be appreciated that the channel response estimation techniquedescribed above may be applied to each of multiple receiver antennas.The procedure would be applied separately for each receiver antenna.Also, one may take into account known channel response components byapplying the techniques of U.S. application Ser. No. 09/234,929.

It is understood that the examples and embodiments described herein arefor illustrative purposes only and that various modifications or changesin light thereof will be suggested to persons skilled in the art and areto be included within the spirit and purview of this application andscope of the appended claims and their full scope of equivalents. Allpublications, patents, and patent applications cited herein are herebyincorporated by reference.

1. An apparatus for transmitting OFDM data, including a plurality offrequency domain data symbols, the transmitter comprising: a converterarranged in operation to convert a plurality of frequency domain datasymbols and a plurality of training symbols in a corresponding pluralityof substantially orthogonal frequency subchannels to a time domain burstfor transmission, the training symbols having a pre-defined value; andtransmitter arranged in operation to transmit the time domain burst,such that a receiver receiving the transmitted time domain burst is ableto use the transmitted training symbols to estimate the channelresponses for the subchannels by converting the received time domainburst into a set of values for the set of subchannels, extracting thetraining symbols from the results of converting the received burst,determining weights for the extracted training symbols based on measuredstatistics indicative of noise and/or interference, and using thedetermined weights to estimate the channel responses for thesubchannels.
 2. An apparatus as recited in claim 1, wherein thetransmitter is a radio transmitter, and wherein the converter of theapparatus includes a digital Fourier transformer that performs aninverse Fourier transform from the frequency domain to the time domain.3. An apparatus as recited in claim 1, wherein the training symbols arepredefined such that at the receiver, the using of the determined ofweights uses a weighted least squares estimation procedure.
 4. A methodof transmitting OFDM data, including a plurality of frequency domaindata symbols, the method comprising: converting the plurality offrequency domain data symbols and a plurality of training symbols in acorresponding plurality of substantially orthogonal frequencysubchannels to a time domain burst for transmission, the trainingsymbols having a pre-defined value; and transmitting the time domainburst, such that a receiver receiving the transmitted time domain burstis able to use the transmitted training symbols to estimate the channelresponses for the subchannels by converting the received time domainburst into a set of values for the set of subchannels, extracting thetraining symbols from the converted received burst, determining weightsfor the extracted training symbols based on measured noise and/orinterference, and using the determined weights to estimate the channelresponses for the subchannels.
 5. A method as recited in claim 4,wherein the transmitting includes wirelessly transmitting using a radiotransmitter, and wherein the converting of the plurality of frequencydomain data symbols and the plurality of training symbols includesdigital Fourier transforming by an inverse Fourier transform operationfrom the frequency domain to the time domain.
 6. A method as recited inclaim 4, wherein the training symbols are predefined such that at thereceiver, the using of the determined of weights uses a weighted leastsquares estimation procedure.
 7. In a receiver of a digitalcommunication system, a method for estimating a channel response for aset of substantially orthogonal subchannels, the method comprising:receiving a time domain OFDM burst formed by converting to the timedomain a plurality of frequency domain data symbols and a plurality offrequency domain training symbols in a corresponding plurality ofsubstantially orthogonal frequency subchannels; converting the receivedtime domain OFDM burst to a set of frequency domain values for the setof subchannels; extracting the frequency domain training symbols fromthe converted received time domain OFDM burst; determining weights forsaid frequency domain training symbols based on a statistical measure ofnoise and/or interference, such that the respective weights provide somemeasure of the corruption by interference and/or noise that respectivefrequency domain training symbols experience; and using the weights andextracted frequency domain training symbols to determine the channelresponse for different subchannels, such that training symbolsexperiencing greater corruption by interference and/or noise have lessinfluence on the determined channel response than training symbolsexperiencing less corruption by interference and/or noise.
 8. A methodas recited in claim 7, wherein the using of the weights includes using aweighted least squares estimation procedure.
 9. A method as recited inclaim 7, wherein the determining of weights includes determining weightsthat are binary valued.
 10. A method as recited in claim 7, wherein thereceiving includes wirelessly receiving, and wherein the converting to aset of frequency domain values includes digital Fourier transforming bya Fourier transform operation.